The present invention relates in general to a voltage-controlled amplifier (VCA) having a minimum gain value and a maximum gain value set to any selected values by fixed resistors, and more specifically, to a gain-matched multichannel audio amplifier wherein each channel employs a JFET in the feedback loop of an operational amplifier.
Amplification of electrical signals by a desired gain can be achieved using operational amplifier (op amp) integrated circuits. By providing negative feedback between the output of the op amp and its inverting input, the gain provided by such an amplifier is controlled by external resistances connected to the op amp as is known in the art. If one or more of the external resistances are variable resistances, the amplifier gain may be varied. Thus, a variable-gain amplifier can be realized by employing a potentiometer in the feedback path between the output and inverting input of the op amp, for example.
It is also known to employ a voltage-controlled attenuator such as a junction field-effect transistor (JFET) operating in its linear or triode region of operation as a variable gain-setting resistance used in conjunction with an op amp. Thus, a gate control voltage supplied to the JFET determines the amplifier gain. However, several disadvantages are associated with certain inherent properties of JFETs. Due to uncontrollable variations in the fabrication process of JFETs, various electrical properties, such as pinch-off voltage V.sub.p, on-resistance R.sub.ds(on), and the resistance curve between R.sub.ds(on) and V.sub.p, have a wide variation from one JFET to another JFET of the same design. The use of a JFET as a voltage-controlled attenuator is strongly influenced by these uncontrolled characteristics, thus limiting the usefulness of JFETs as attenuators. Testing of JFETs after manufacture to sort them according to similar characteristics greatly increase the costs associated with using JFETs as attenuators. Furthermore, sorting cannot eliminate all variability in the JFET characteristics.
Variations in the electrical properties of JFETs create problems if JFET-controlled op amps are to be used in multichannel audio amplifiers (e.g., audio limiters) which require that separate channel gains closely track each other. Even sorted JFETs do not provide sufficiently matched characteristics to obtain the required gain tracking.
In order to improve the characteristics of an amplifier using a JFET in the feedback path of an op amp, a fixed resistor may be combined in parallel with the JFET. For example, in an Application Note 125 from National Semiconductor Corporation, an op amp gain control circuit is shown which employs a fixed resistor connected in parallel with a JFET in order to get a minimum gain value independent of the JFET characteristics. The application note teaches that gain tracking between different amplifiers requires monolithic dual or quad JFETs. However, the use of discrete JFETs may often be preferred because of lower price, flexibility in packaging (i.e., several smaller JFETs rather than one larger one), and the fact that even the best monolithic dual or quad JFETs exhibit variation in pinch-off voltage of a few tenths of a volt (which is enough to cause serious mismatching in a multichannel amplifier).
In a multichannel audio amplifier, it would be desirable to match all the minimum and maximum gains at fixed values independent of the JFET characteristics in order to prevent errors in stereo imaging. But even using dual or quad JFETs as suggested in the Application Note 125, the maximum gain is uncontrolled. The gate control signal would have to be precisely controlled so that the desired amount of gain would be obtained and so that a maximum desired gain is not exceeded. However, the precise voltage to be generated cannot be known in advance because of the uncertainty of the actual resistance curve of a particular dual or quad JFET. Thus, prior art amplifiers employing JFET controlled op amps perform in unpredictable ways, making them unsuitable for various applications such as a multichannel audio limiter.